Seismic sensor recording system

ABSTRACT

A sensor system for deployment on or close to the seabed in marine seismic surveys includes a central hub, and a plurality of arms coupled to the central hub. Each arm has a degree of freedom of movement with respect to the central hub. The system further includes at least one seismic sensor mounted to each of said arms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/309,007, filed on Nov. 4, 2016, which is the National Phase of PCTInternational Application No. PCT/EP2014/059391, filed on May 7, 2014,all of which are hereby expressly incorporated by reference into thepresent application.

TECHNICAL FIELD

The present invention relates to a seismic sensor recording system foruse in conducting marine seismic surveys. The invention also relates toa method of using such a system.

BACKGROUND

In the context of marine seismic surveys, two types of seismic waves areof interest, namely P waves and S waves. P waves, or Primary waves, arecompressional waves that are longitudinal in nature. These are pressurewaves that can travel through any type of material including fluids. Swaves, or Secondary waves, are shear waves that are transverse in natureand cannot travel any distance through fluids. They travel more slowlythrough solid materials than P waves, hence the name (“Secondary”). As Swaves cannot travel through fluids, they can only truly be detected byreceivers that are mechanically coupled to the seabed. Sophisticatedprocessing techniques have been developed to make use of detected S andP waves to image subsea regions and in particular to detect and monitorhydrocarbon bearing formations.

Whilst, traditionally, P waves have been detected using arrays ofsources and receivers towed in the water, both P and S waves can bemonitored by measuring two physical effects at the seabed, namelypressure and particle velocity or particle acceleration. These measuredphysical effects may be analyzed using complex algorithms in order todetect and separate the P and S waves. Traditionally, seismic seabedsurveys have been conducted using arrays of so-called 4 c sensors, eachof which monitors four components, namely pressure and three orthogonalcomponents of particle velocity (x, y and z), or particle acceleration,using a single hydrophone and three orthogonally-oriented geophones.More recently, it has been appreciated that additional data—includingpressure derivatives in the horizontal plane (x and y directions) andthe particle velocity derivatives in the horizontal plane (x and ydirections)—can prove valuable in monitoring the P and S waves,resulting in higher quality (e.g. higher resolution) data and addedvalue in subsurface mapping. [The terms “gradients” and “derivatives”are used interchangeably in the technical literature.]

It is noted that the horizontal particle velocity (in the water column)can be, and in practise normally is, derived from the horizontalpressure gradient measured at the seabed. Furthermore, the horizontalparticle velocity's horizontal gradient can be derived from thederivative of the pressure gradients, that is the second orderhorizontal pressure gradient, and so forth.

To obtain additional data to improve the quality or value of the P-wavefield data , so-called 6 c sensors are employed to measure sixcomponents, namely; pressure (p) and its first order spatial derivativesin the horizontal plane (dp/dx, dp/dy), and vertical particle velocity(Vz) and its spatial derivatives in the horizontal plane(dVz/dx,dVz/dy). In some cases, even more complex sensors may be used,e.g. 10c sensors to collect the 6 c data plus four second orderderivatives. These sensors do not necessarily need to be at the seabed,and could in principle be positioned anywhere in the water column.However, in order to measure S-waves, seabed coupled horizontalgeophones or accelerometers are needed. These sensors are included astwo of the components in traditional “4C seismic seabed recorders”. Herethe four components (4C) are: pressure, vertical particle velocity andthe two orthogonal horizontal particle velocity sensors. When 6C and/or10C sensors are combined or integrated with one or more seabed coupled4C sensors, additional data is then available for improving the dataquality of both S-wave and P-wave data.

FIG. 1 illustrates schematically two possible 6 c sensor configurations.On the left is shown a configuration comprising 3x2 c sensors, eachcomprising a hydrophone and a vertically oriented geophone. On the rightis shown a configuration comprising 6xP sensors, each comprising asingle hydrophone (nb. it is known that vertical particle velocity canbe measured by making two separate vertically spaced pressuremeasurements).

A number of texts cover the principles of acquisition of marine seismicdata (e.g., Sheriff and Geldart, 1995; Ikelle and Amundsen, 2005). Thereare several configurations of source and receiver distributions; thosecommonly used for petroleum exploration are (1) towed-streameracquisition, where sources and receivers are distributed horizontally inthe water column near the sea surface; (2) ocean-bottom seismic (OBS)acquisition, where the sources are towed in the water column and thereceivers are on the seafloor; and occasionally (more rare) (3)vertical-cable (VC) acquisition, where the sources are towed near thesea surface as in towed-streamer and OBS acquisition but the receiversare distributed in the water in a vertical array.

A particular case of the OBS acquisition involves the use of OceanBottom Nodes (OBNs), rather than the ocean bottom cables. OBNs aretypically battery powered, cableless receivers typically deployed one byone in deep water, or attached to a wire or rope for deployment inshallower waters, whatever makes the operations most safe and efficient.OBNs are especially suited for use in relatively congested waters wherethe towing of streamers and/or deployment of ocean bottom cables isdifficult. OBNs are typically deployed and recovered by Remote OperatedVessels (ROVs), using free fall systems and acoustic release tofacilitate recovery, or using “nodes on rope” techniques where multiplenodes are attached to a rope with an acoustic release buoy at the end.These approaches are traditionally used to detect data that consists ofboth P and S waves. It should also be noted that there are significantadvantages to collecting data (P waves) at or close to the seabed whererecording conditions are quiet, being shielded from sea currents, andwhere conditions are good for low frequency data recorded by particlevelocity sensors or accelerometers.

WO2011/121128 describes a method of providing seismic data (such asmarine seismic data). A seismic source is actuated at a plurality ofsource locations. For each source location, a multicomponent seismicmeasurement is performed at at least one receiver location. Areconstructing method is applied to each multicomponent measurement toobtain additional data corresponding to source locations additional tothe source locations at which the source was actuated. The additionaldata are output and/or used. WO2011/121128 proposes, by way of example,that this approach may be used in the context of OBN/OBS acquisition,i.e. where multicomponent (6 c) receiver nodes are located on the seabedand the sources are towed in the water column by a surveying vessel.

Commercial Oil and gas discoveries are typically found in sedimentarystructures defined as “traps”, where porous rocks are covered by tightcap rocks. The structures are visible on seismic images due tovariations in elastic properties of the rocks. P and S wave derivedimages may have different expressions, because their response isdetermined by different elastic properties (shear stiffness and normalstiffness) and may produce images that can be both supplementary and/orcomplementary. For example, S waves may more easily “see through”overburden sediments containing gas, whereas P waves may be completelyattenuated. Furthermore, S waves may be more responsive to fluidoverpressure and associated Geohazards. On the other hand, P waves aremore sensitive to fluid type (distinguish gas, oil, water) than are Swaves. Using the combination of P and S wave responses, one can improvethe overall geological and geophysical interpretation of the data,providing a more accurate estimate of location, size and volume (andpressure) prediction, and type of fluids presents in the reservoirs.

In order to produce high quality S and P images of the subsurface,advanced data processing of the recorded data is needed in order tofilter out noise and “beam-form” or migrate the seismic energy to theright location (to the image point). Traditionally P and S data areimaged separately, and one assumes (requires) that the P-wave data setis free of S waves (also free of S to P converted data) and the S-wavedata set is free of P waves. This may not be the case in practice, andtherefore the results may be compromised.

Traditionally, the seismic industry relies upon processing/imaging stepsto try to “wash out” and suppress any P wave/S wave crosstalkinterference. Clearly, reducing the levels of noise in the input S and Pwave data would improve the final image/or inversion results (for agiven amount of effort/data size input and set of processing steps).Cleaner S and P input data, also would make processing/imaging/inversionusing the wave equation more efficient, because a coupled solution(using full elastic formulation) may be split into separate processes,and run more efficiently with simpler formulations (for example scalarformulations).

A problem encountered with OBS systems is the interference that occursbetween the two types of waves. For example, a detector mechanicallycoupled to the seabed and configured to detect S waves will pick up theeffects of P waves propagating in the seabed. Although it may bepossible to remove much of the effects of the early P waves by filteringbased upon arrival time (P waves propagate faster through the subseaformation than do S waves) and apparent speed (or so called “moveout”),not all of the effects can be removed, due to mixing with later Parrivals, for example as a result of reflections from differentinterfaces, ringing in the source signal, and overlapping P and S energyin time due for example to P-S conversion and reflections at or close tothe seabed. Conversely, a detector located in the water just above thesubsea surface and configured to detect the effects of P waves may beinfluenced by S waves. Although S waves do not propagate through thewater, there will be some conversion of S waves and surfacewaves/interface waves (Scholte wave; S-wave travelling along the seabed)to P waves at the seabed. It is desirable to remove the effects of suchconverted S waves from the data collected by the P wave detector andremove the effects of P-waves on the S-detector.

U.S. Pat. No. 5,894,450 describes an oceanographic sampling systememploying an array of underwater vehicles. U.S. Pat. No. 6,842,006describes a sea-floor electromagnetic measurement device for obtainingunderwater measurements. US2012/0067268 describes a subsea verticalglider robot for use in oceanographic research. US2006/0256652 describesa method of acquiring seismic data and which involves deploying an arrayof seismic receivers dropped onto the seabed. US2013/0058192 describesan ocean bottom seismic cable recording apparatus. US2013/0081564describes a deployment and recovery vessel for an autonomous underwatervehicle for marine seismic surveys.

SUMMARY

According to a first aspect of the present invention there is provided asensor system for deployment on or close to the seabed in marine seismicsurveys. The system comprises a central hub, and a plurality of armscoupled to the central hub, wherein each arm has a degree of freedom ofmovement with respect to the central hub. The system further comprisesat least one seismic sensor mounted to each of said arms.

The sensor system may be, for example, and Ocean Bottom Node or anotherwise seabed anchored node.

Considering further the sensor system, each arm may coupled to thecentral hub so as to allow the arm to pivot relative to the central hub.The arms may be pivotal between a near vertical pre-deployment positionto a near horizontal deployed position, the sensor system comprising anarm release mechanism for facilitating movement of the arms from thepre-deployment position to the deployed position when the central hub islocated on or close to the seabed. Alternatively, the arms may be freelypivotable over an angular range sufficient to allow all arms to settleonto the seabed when the central hub is located on or close to theseabed.

The sensor system may comprise a data recorder located within thecentral hub, the data recorder being coupled to each of the sensors inorder to receive seismic data therefrom and, optionally, a clock locatedwithin the central hub to enable synchronization of data received fromthe sensors.

The sensor system may comprise a data recorder co-located with the oreach seismic sensor and, optionally, a clock co-located with the or eachseismic sensor. The sensor system may comprise at least one seismicsensor located within or in close proximity to said central hub.

Each seismic sensor mounted in each of said arms may be configured to bein mechanical contact with the seabed when the sensor system is deployedwhilst substantially not being in mechanical contact with the arm orbeing secured to the arm by means of non-rigid coupling. The or eachseismic sensor may be mounted at a peripheral end region of an arm.

According to a second aspect of the present invention there is provideda sensor system deployment on or close to the seabed in marine seismicsurveys. The system comprises a central hub configured to sit on orclose to the seabed, a mast projecting upwardly from the central hub,and a plurality of seismic sensors attached to the mast at at least twovertically spaced locations. The system may comprise a first set ofseismic sensors attached to said mast at a first, lower level, and asecond set of seismic sensors attached to said mast at a second, upperlevel.

Each said set of seismic sensors may comprise a set of pressure sensorsspaced angularly around the mast, each being at a radial offset positionwith respect to the mast. The seismic sensor may be attached to the mastat one or both of the first and second levels, substantially at aradially central location, for example comprising at least one seismicsensor attached to said mast at a position between said upper and lowerlevels.

The sensor system may comprise a first set of accelerometers at saidupper level and a second set of accelerometers at said lower level, eachsaid set being located within a neutrally balanced housing coupled tosaid mast.

The system may comprise a data recorder, and optionally a clock, locatedwithin the central hub, the data recorder being electrically coupled toeach of the sensors in order to receive seismic data therefrom.Alternatively, the system may comprise a data recorder, and optionally aclock, co-located with each seismic sensor.

The sensor system according may comprise at least one electromagneticsensor. Each electromagnetic sensor may comprise one or more electrodeslocated in a peripheral end region of each of said arms.

According to a third aspect of the present invention there is provided asensor system for deployment on or close to the seabed in marine seismicsurveys. The system comprises a substantially rigid and planar frame,and a plurality of seismic sensors mounted to the frame at respectiveperipheral locations. Optionally, a further sensor may be mounted at acentral location.

The frame may be substantially triangular or quadratic and a seismicsensor is mounted at or close to each of the points of the triangular orquadratic frame. Each seismic sensor may be mounted to the frame bymeans of a substantially flexible coupling mechanism. The couplingmechanism may comprise one or more wires, ropes, or elastic bands.

According to a fourth aspect of the present invention there is provideda marine seismic survey system comprising one or more ropes, a ropedeployment system for deploying the or each rope on or close to theseabed, and a multiplicity of sensor systems according to the abovethird aspect of the invention. The marine seismic survey system furthercomprises an attachment mechanism for attaching and detaching the sensorsystems to the rope(s) during deployment.

According to a fifth aspect of the present invention there is providedan apparatus for collecting sensor nodes used in a marine seismicsurvey, where the nodes are configured to ascend towards the sea surfacefollowing use in a seismic survey, the apparatus comprising a funnel fordeployment in the water and being configured to receive and captureascending sensor nodes.

The apparatus may comprise a detector for detecting location relatedsignals emitted by sensor nodes and a transmitter for sending receivedlocation related signals to a vessel positioning system of a vessel onwhich the apparatus is located.

The apparatus may comprise a node deployment mechanism configured todeploy nodes into the water through said funnel.

According to a sixth aspect of the present invention there is provided avessel comprising the apparatus of the above fifth aspect of theinvention.

According to a seventh aspect of the present invention there is providedmethod of collecting sensor nodes used in a marine seismic survey. Themethod comprises causing a sensor node to ascend through the watertowards the surface, causing the sensor node to enter a collectionsystem deployed on or close to the sea surface, via a funnel submergedin the water, capturing the sensor node within the collection system,and moving the captured sensor node to a storage location.

The step of causing the sensor node to enter a collection system maycomprise estimating a surface arrival location for the ascending sensornode, and causing said funnel to move to approximately said surfacearrival location.

Where the collection system is located on or within a vessel, the stepof causing said funnel to move to approximately said surface arrivallocation comprising maneuvering the vessel to put the funnelapproximately at said surface arrival location.

The step of estimating a surface arrival location for the ascendingsensor node may comprise receiving a wireless signal, for example anacoustic signal, transmitted by the sensor node and processing thatsignal to estimate the surface arrival location.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically two alternative 6C sensorconfigurations;

FIGS. 2A and 2B illustrate a tripod OBN system in a deploying anddeploying configuration;

FIG. 2C show a detail of the OBN system of FIGS. 2A and 2B, showing inparticular a sensor;

FIG. 3 shows an alternative OBN system in a deployed state;

FIG. 4A shows the OBN system of FIG. 3 in an undeployed state;

FIG. 4B shows various deployment steps for the OBN system of FIGS. 3 and4A;

FIG. 5 shows a further alternative OBN system;

FIG. 6 shows a still further alternative OBN system;

FIG. 7A shows a still further alternative OBN system comprising atriangle support frame;

FIGS. 7B and 7C show two alternative node-on-a-rope deployment systems;

FIG. 7D shows a “ski-lift” deployment system for the OBN system of FIG.7A;

FIGS. 8 and 9 show respectively topside and bottomside views of an OBNdeployment system;

FIGS. 10a and 10b illustrate two procedures for laying ropes andemploying a forward moving vessel; and

FIG. 11 illustrates an alternative procedure for laying ropes using asideways advancing vessel.

DETAILED DESCRIPTION

It will be appreciated from the discussion presented above that the useof Ocean Bottom Nodes (OBNs) operating as receivers in marine seismicsurveys is very desirable as it allows both S and P wave data to becollected with relatively high signal-to-noise rations. The receiversshould be able to operate in both shallow and deep water (>2000 m). Thesurvey areas can be large and may involve shuffle/roll of receiversalong the seabed as shooting progresses. Large numbers of receivers (onthe order of several thousand) may be deployed in a large surveycampaign.

The node spacing of a 3D seabed seismic survey for deeper targets istypically 200-400 m, denser for shallower targets. It should be possibleto land receivers on the seabed to an accuracy within 10%-20% of thereceiver separation distance. Receivers may be provided with onboardpositioning systems to ensure that the node spacing (landing position)is within the tolerances. The actual seabed position of the receiverscan be calculated from the seismic shots performed during the survey.The heading, that is orientation, of the nodes can be arbitrary, as longas the orientation can be determined from the shots (or using otherinstruments). [When using nodes on wire or rope, the inline spacingbetween the nodes typically is 25 m to 50 m. The spacing between linesmay be 150-300 meters.]

Embodiment 1

FIGS. 2A and 2B illustrate schematically an Ocean Bottom Node (OBN)receiver system 1. More particularly, FIG. 2A illustrates the system asit is in the process of being lowered into the water, e.g. from a surveyvessel, whilst FIG. 2B illustrates the system once it has been loweredto, and settled onto, the seabed.

The system comprises a frame 2 having three arms 3 a-c attached to acentral support hub 4. The arms are attached to the hub by respectivehinges 5 that allow the arms to pivot relative to the hub to somelimited extent, e.g. zero to −30 degrees relative to the horizontal. Thearms are equally spaced around the central hub, i.e. spaced by 120degrees.

At the end of each arm 3 a-c there is provided a 4c sensor 6 a-c thatmay have a known construction and mode of operation. For example, thesensors may be Geospace Ocean Bottom Recorders (OBX™) incorporating ahydrophone and three orthogonally oriented geophones. As shown in FIG.2C, which shows a detail of the system 1, the sensors 6 a-c are attachedwithin respective arms by wires or ropes or elastic bands 7 that areprovided with a small degree of slack, but which are otherwise tightenough to retain the sensors centralized within the arms, avoidingdirect contact between the arms and the sensors. This allows the sensorsto settle into contact with the seabed once the sensor system is fullydeployed, whilst of course retaining the sensors within the frame duringthe deployment procedure. This contact with the seabed is important asit allows data associated with both P and S waves to be collected by thesensor system. The sensors may incorporate clocks and/or data recorders.

This mechanical system decouples the sensors from the frame structure,due to the slack on the wires (or ropes or elastic bands), once it haslanded on the seabed, limiting potential crosstalk between theindividual nodes via the frame and/or avoiding picking up noise from themotion of the frame itself. It will also be noted that the hingedattachment (soft joints) of the arms to the central supportstructure—providing a tripod like structure—further assists in reducingpotential motion (e.g. resonances induced in the frame from sea currentsor due to instability on a rough seabed) via the arms, and ensuresappropriate contact between the sensors and the seabed particularlywhere the seabed is uneven or rocks or other projections are present.

It will be appreciated that the system of FIG. 2, comprising as it doesthree regularly spaced 4c sensors, is able to measure pressure andvertical particle velocity in three orthogonal directions (x,y and z),at three known locations. This in turn allows the calculation of thehorizontal spatial derivatives of the pressure and particle velocityincluding the 6 c components to be derived, and in particular thegradients dp/dx, dp/dy and dVz/dx,dVz/dy.

A central portion of the system may be a void as shown in the Figures.However, a further sensor, such as a 4C unit, may be added at the centreof the structure, e.g. within a central the hub. A data recorder and orclock may also be incorporated into this hub.

The system may be configured to allow the second order horizontalspatial derivatives of the pressure and particle velocity to becalculated, to improve value even more (allowing even betterinterpolation and quality improvement in data processing and imaging).

Embodiment 2

FIG. 3 illustrates schematically an alternative Ocean Bottom Node (OBN)receiver system. This system 10 comprises four foldable sensor arms 11a-d that in situ are stretched out on the seafloor and connected to abuoyant central recording hub 12 with an internal acoustic releasesystem 13 and anchor plate 14. The arms are pivotally attached to therecording hub 12 via respective hinges 15. Whilst FIG. 3 illustrates theOBN receiver system in a deployed state, FIG. 4A illustrates the systemin a pre-deployment state in which the arms 11 a-d are folded to a nearvertical position for engagement at their upper ends with a deploymenttool 16.

The sensor system 10 may be lowered to the seabed, or to a positionclose to the seabed, and a trigger rope 17 (see FIG. 4) may be used torelease the unit allowing it to fall through the water and land on theseabed with its arm unfolded. Typically the release system willincorporate a hydro-acoustic transponder to give the position of theunit before it is released. When less accurate target drop positions areallowed, the system may be dropped from a position higher up in thewater column, or even from air into the water. The system is made suchthat the drag of the arms and the speed of the decent through the watercauses the arms to remain folded in an upright position during thedecent, with the arms subsequently unfolding automatically due to theirown weight when the central hub hits the seabed and motion stops. Thisdeployment process is illustrated in FIG. 4B. A 2c sensor 18 a-d islocated at the end of each arm, the sensor comprising a hydrophone and avertical geophone. In addition, a further 2c sensor 18 f is located inthe recording hub 12, with all sensors being mechanically coupled to theseabed. Instead of 2c sensors, all or some of the five sensor units maybe 4c sensors (each comprising a pressure sensor and three orthogonalgeophones or accelerometers).

In order to facilitate release of the system 10 from the seabed aftercompletion of a survey, the recording hub 12 comprises a buoyant housing18 to which the foldable arms are attached. The housing 19 is in turncoupled to a “clump weight” or anchor plate 20 that sits beneath thehousing. This coupling is via an attachment 21 that includes anacoustically activated release mechanism 22. Following completion of asurvey an acoustic release signal can be transmitted to the system 10(e.g. from a survey mechanism or acoustic source lowered into the water)to cause the buoyant housing 19 and the attached arms etc to be releasedfrom the anchor plate 20, allowing the system 10 to float to the surfacefor subsequent collection. Alternatively, the release system releases abuoy that floats up to the surface with a rope attached to the seabedunit.

The sensor system 10, comprising five regularly spaced 2c sensors (or 4csensors), again allows the following 6 c components to be measured orderived at the seabed: p, dp/dx, dp/dy and V, dVz/dx,dVz/dy.

Embodiment 3

FIG. 5 illustrates a 6 c OBN sensor system 20 that conforms to theprinciples of the system illustrated generally in FIG. 1, right handview. However, rather than having three hydrophones (pressure sensors)at each level (upper and lower), the system comprises five hydrophonesat each level, four 21 a-d, 22 a-d arranged at the ends of respectiveextension arms, and a single centrally located hydrophone 23 a,b. Inaddition, a further hydrophone 24 is provided at a central location,intermediate the upper and lower hydrophone arrays. The various sensorsare attached to a rigid, vertically extending mast (although this mastmay be, for example, a rope supported at its upper end by a flotationelement).

As with embodiment 2, this third embodiment comprises a central, buoyanthub 25 to which the hydrophone arrays are secured. The central hub 25 isin turn releasably coupled to an anchor plate 26 via an acoustic releasemechanism 27. Again, as with embodiment 2, the central hub and attachedsensor structure may be floated back to the surface by activating theacoustic release mechanism 27.

Each hydrophone in the array is configured to measure pressure, and eachmay be configured to record independently of the other hydrophonesindividually. Alternatively, pairs of hydrophones may be hardwired inselected pairs. The known geometrical distribution (individual distancesand relative orientation) of the sensors makes it possible to measure orcalculate the first and second order spatial derivatives of pressure, inall three directions (x,y,z). Note that the first derivative of pressureis density times acceleration (or particle velocity, if integrated overtime). That is, the recording gives particle acceleration (or particlevelocity) and its first order spatial derivative in all directions. Aslong as the structure of the arms and supports are relatively “soft”, asis the intension of the construction, a sensor will be detached from themotion of the seabed (“non-coupled”) and the data will representpressure and particle motion of the water only, unaffected by thepresence of the recorder structure itself. The structure itself isdesigned to minimize interaction between the structure and the pressureand pressure gradients in the water, by selecting rigid but thin andsmall dimensions for the support, whilst keeping the sensors atsufficient distance from the major acoustic wave reflecting objects ofthe unit, including the buoyant hub and anchor.

In addition to the hydrophone array, one may add a 4C sensor at the baseof the structure, to record shear wave data at the seabed. The combineddata set may be used to improve the data quality for both P-waves andS-waves.

Embodiment 4

A further sensor configuration 30 is illustrated schematically in FIG.6. This configuration is similar to that of Embodiment 3, except thatthe hydrophone arms of embodiment 3 are replaced with two suspended 4Csensors. Each 4C sensor comprises one hydrophone and three orthogonalaccelerometers or geophones, e.g. micro electro-mechanical systems(MEMS) accelerometers. These are contained within upper and lowerneutrally balanced spheres 31, 32 that are suspended within a frame 33,supported by thin ropes or thin elastic rubber band in order to preventthe sensors from floating away from their designated positions anddirections whilst allowing them to be acoustically decoupled from thesupport structure. The central recording hub 34 may contain a singlehydrophone and three orthogonally oriented (MEMS) accelerometers,coupled to the seabed to measure shear waves. It may also contain aseparate tilt meter to measure the inclination of the structure (if theMEMS system cannot do that by itself).

It will be appreciated that the sensor system of FIG. 5 is able tomeasure P, Vx, Vy, and Vz within the water, at two vertically spacedapart locations. Hence, it is possible to derive the vertical gradientof the horizontal particle velocity. An analysis of the relevantmathematics shows that the vertical gradient of the horizontal particlevelocity is analogous to the horizontal gradient of the verticalparticle velocity (using curl V=0).

It is known that interesting information concerning subsea structurescan be obtained using electromagnetic (EM) surveys. The informationobtained from such surveys may be used to enhance or compliment imagesobtained using marine seismic surveys. It will be appreciated that EMsensors required to conduct an EM survey may usefully be integrated intosensor systems of the type described above.

Embodiment 5

FIG. 7a illustrates a further sensor system 40 that is suitable fordeployment via a rope or wire (a special “triangle nodes on wire”arrangement). The triangle node sensor system comprises a rigid frame41, with three or more 4c sensors 42 a,b,c attached to it using thewire-principle (described above) in order to decouple the nodes from thestructure. The Triangle nodes 40 can either be attached to the wire 43along the centre line or along one side (as a flag line). The bestconfiguration depends on how the nodes are attached and detached fromthe wire, the behaviour during lay out and recovery, and the seismicde-coupling from the wire during recording. FIGS. 7b to 7d showndifferent options for deployment of this system:

FIG. 7b —triangle with wire centre line attachment

FIG. 7c —flag line attachment

FIG. 7d —“ski-lift” principle used in a launch and recovery system forthe “flag line attachment”.

The “flag line” fixture is the preferred configuration as it will allowfor simpler attachment to the wire using spring loaded grips (as forski-lifts); for both alternatives the triangle can rotate around thewire, thus the suspended position of the node will depend on gravity butthe drag forces will be control the orientation during deployment. Thus,for the flag line concept, it is virtually impossible for the trianglesto penetrate into the seabed. Rather, they will naturally tend to flipto the side. The venting area is also removed in the outer section inorder to ensure that this part will be flipped up by the drag when thewire and node is laid down on the seabed. Basin or field testing ishowever recommended in order to verify and optimize the hydrodynamicbehaviour and design.

The nodes are attached to the main laying wire acting as a long “fishingline” for the nodes. The main steel wire has a cross-section to givesufficient on-bottom weight. The wire may possibly be prepared withmating sleeves. The use of spring loaded grips to allow attachment tobare wire may also be possible. In principle only one fixed grip isrequired; the other corner can have a loose grip ensuring that the nodestays parallel to the wire. The nodes are first suspended from a storagerail where an automatic shuttle feeder latches the nodes on and off thewire. Recovery is performed by running the line and machinery in theopposite direction. This arrangement allows for continuous wireinstallation/recovery without interruption and manual handling.

Similar triangular sensor systems may be configured for deployment bymeans of an ROV.

OBN Surface Collection System

When using nodes suited for individual deploy and/or recovery from theseabed by release systems and buoyancy aided recovery, a safe andefficient surface deploy and collect system is required. There will nowbe described such a system that is referred to here as “The TeleporterLaunch and Rovery System”. This is a system in which nodes are releasedand hooked up under the sea surface. The nodes may be fed to the LaunchAnd Recovery System (LARS) on conveyor belts, hooked up to the hoistingwinch, and dragged into the “Teleporter” chamber (the upper part of thelaunch and recovery tube).

FIG. 8 shows two topside views of the Teleporter Launch and Recoverysystem 50 that will typically be mounted on the deck 51 of a supportvessel. A sensor node 52 is hooked up with a special release and dockingtool 53 and hoisted down to the submerged end of the launch and recoverytube 54. The tool releases the node by means of topside command(automatic or manual), allowing the sensor node 52 to drop through thewater.

FIG. 9 is a bottomside view of the Teleporter Launch and Recoverysystem. During recovery a sensor node 52 must enter into a guide funnel55, (a) and (b), progressing upwards until it stops at docking positionbelow the sea surface (c). The node 52 is centralized inside the launchand recovery tube 54 by means of the protection and guide bars aroundthe rudders. The release and docking tool (also with guide bars forcentralization inside the tube) can be in position or hoisted down fordocking to the node, which then is hoisted up to the “Teleporter” anddeployed on the conveyor belt. The docking and release position ismonitored by a subsea camera. If required the hoisting winch or even thecomplete launch and recovery tube can be heave compensated.

The funnel 55 can be equipped with a special homing device in order tosimplify navigation into the funnel. One embodiment would involve ahydro-acoustic navigation transponder on each node, with the dynamicpositioning system of the vessel being locked to the node allowing thevessel to be steered to a horizontal directly above the ascending node,ensuring that the node finally “hits” the funnel. To increaseefficiency, several nodes may be in transit from the seabedsimultaneously, surfacing just with enough delay to allow the vessel tobe correctly repositioned between each node.

Continuous launch and recovery operations may require “production line”arrangements on deck, with data down-load, synchronization,re-programming and checking of the nodes. Two “Teleporters” may beprovided on either side of the vessel, one for launch and the other forrecovery.

As a backup, facilities for conventional crane pick-up should beavailable on the vessel in case a node is unable to reach the funnelentry point. Nodes should have surface tracking facilities andflashlights to make them easier to locate on the surface.

Referring now to FIG. 10a , this illustrates a method of deployingcomplex sensor systems on the sea bed. According to this method a vessel60 advances in a conventional manner whilst laying three ropes, 61 a toc, in parallel and closely spaced. This spacing might be for example 2to 15 meters. Each rope supports at regular or semi-intervals sensors 62such as a 2C or 4C sensor. The sensors are aligned such that, once laid,the sensors form spaced clusters 63 of 2C or 4C sensors, with threesensors in each cluster. Each cluster therefore provides a seabedmulticomponent sensor system such as the 6C sensor system describedabove. Once laid, the exact sensor location, spacing and orientation canbe determined from the recorded seismic data itself (using severalshots) or by hydro-acoustic systems incorporated on the nodes or seabedcable/rope. In an alternative configuration, a vessel may lay only twoparallel ropes, with one rope having double the number of sensors. Thisis illustrated in FIG. 10 b.

This approach to deploying multicomponent sensor systems is one that canbe implemented with relative ease relying as it does on existingtechnologies used for laying single strings of 2C or 4C sensors.

FIG. 11 illustrates an alternative approach to deploying these sensorsystems and which relies upon the laying vessel 60 advancing sideways asthe ropes are laid.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiments withoutdeparting from the scope of the present invention.

1. A sensor system for deployment on or close to a seabed in marineseismic surveys, the sensor system comprising: a central hub configuredto sit on or close to the seabed; a mast projecting upwardly from thecentral hub; and a plurality of seismic sensors attached to the mast atat least two vertically spaced locations.
 2. The sensor system accordingto claim 1, further comprising a first set of seismic sensors attachedto said mast at a first, lower level, and a second set of seismicsensors attached to said mast at a second, upper level.
 3. The sensorsystem according to claim 2, each of the first and second sets ofseismic sensors comprises a set of pressure sensors spaced angularlyaround the mast, each pressure sensor being at a radial offset positionwith respect to the mast.
 4. The sensor system according to claim 2,wherein at least one of the seismic sensors is attached to the mast atone or both of the first and second levels, substantially at a radiallycentral location.
 5. The sensor system according to claim 3, wherein atleast another one of the seismic sensors is attached to said mast at aposition between said upper and lower levels.
 6. The sensor systemaccording to claim 2, further comprising a first set of accelerometersat said upper level and a second set of accelerometers at said lowerlevel, each set of accelerometers being located within a neutrallybalanced housing coupled to said mast.
 7. The sensor system accordingclaim 1, further comprising a data recorder, and a clock, located withinthe central hub, the data recorder being electrically coupled to each ofthe seismic sensors in order to receive seismic data therefrom.
 8. Thesensor system according to claim 1, further comprising a data recorder,and a clock, the data recorder and clock being co-located with eachseismic sensor.
 9. A sensor system for deployment on or close to theseabed in marine seismic surveys and comprising: a substantially rigidand planar frame; and a plurality of seismic sensors mounted to theframe at respective peripheral locations.
 10. The sensor systemaccording to claim 9, wherein said frame is substantially triangular orquadratic and one of the seismic sensors is mounted at or close to eachof the points of the triangular or quadratic frame.
 11. The sensorsystem according to claim 9, wherein each seismic sensor is mounted tothe frame by means of a substantially flexible coupling mechanism. 12.The sensor system according to claim 11, wherein said coupling mechanismcomprises one or more wires, ropes, or elastic bands.
 13. The sensorsystem according to claim 9, wherein at least one of the seismic sensorsis mounted to the frame at a central location of the frame.
 14. A marineseismic survey system comprising: one or more ropes; a rope deploymentsystem for deploying the or each rope on or close to the seabed; amultiplicity of sensor systems according to claim 9; and an attachmentmechanism for attaching and detaching the sensor systems to the one ormore ropes during deployment.
 15. An apparatus for collecting sensornodes used in a marine seismic survey, where the nodes are configured toascend towards the sea surface following use in a seismic survey, theapparatus comprising a funnel for deployment in the water and beingconfigured to receive and capture ascending sensor nodes.
 16. Theapparatus according to claim 15, further comprising: a detector fordetecting location related signals emitted by sensor nodes; and atransmitter for sending received location related signals to a vesselpositioning system of a vessel on which the apparatus is located. 17.The apparatus according to claim 15, further comprising a nodedeployment mechanism configured to deploy the nodes into the waterthrough said funnel.
 18. A vessel comprising the apparatus of claim 15.19. A method of collecting sensor nodes used in a marine seismic survey,comprising: causing a sensor node to ascend through water towards thesurface; causing the sensor node to enter a collection system deployedon or close to the sea surface, via a funnel submerged in the water;capturing the sensor node within the collection system; and moving thecaptured sensor node to a storage location.
 20. The method according toclaim 19, wherein said step of causing the sensor node to enter thecollection system comprises: estimating a surface arrival location forthe ascending sensor node; and causing said funnel to move toapproximately said surface arrival location.
 21. The method according toclaim 20, wherein said collection system is located on or within avessel, and said step of causing said funnel to move to approximatelysaid surface arrival location comprises maneuvering the vessel toposition the funnel approximately at said surface arrival location. 22.The method according to claim 20, wherein said step of estimating thesurface arrival location for the ascending sensor node comprisesreceiving a wireless signal, transmitted by the sensor node, andprocessing the wireless signal to estimate the surface arrival location.23. A method of deploying multicomponent sensor systems for use in amarine seismic survey, the method comprising: whilst advancing a vesselacross a surface of water, laying onto the seabed from the vessel two ormore closely spaced ropes, the two or more ropes being laidsubstantially parallel to one another, and each rope having coupled toit at spaced intervals a plurality of seismic sensors; and configuringthe deployed seismic sensors such that each cluster of adjacent sensorson the two or more ropes operates as a single multicomponent sensorsystem, giving rise to multiple single multicomponent sensor systemsalong the length of the deployed ropes.
 24. The method according toclaim 23, wherein the ropes are spaced at a distance of 1 to 25 meters.25. The method according to claim 23, further comprising repeating thelaying and configuring steps multiple times in order to deploy acorresponding multiple of lines of multicomponent sensor systems. 26.The method according to claim 23, further comprising laying at least twoclosely spaced ropes, wherein each sensor is at least a 2C or 4C sensorand each said single multicomponent sensor system is at least a 6Csensor.